For the measurement of temperatures, especially with electrical or electronic instruments, various basic principles have been employed. For example, it is known to measure temperatures by detecting the potential difference or emf produced by a junction of dissimilar metallic and/or semi-conductive temperature-sensitive elements such as thermocouples or thermopiles. Such systems produce an emf which is a function of temperature and may be used for a wide variety of temperature-measuring purposes.
However, when such thermoelements are employed for the measurement of temperatures above about 1000.degree.C or when the measuring instruments or sensor is to be located in an environment which may be destructive to the sensor, the accuracy of the measurement leaves much to be desired and considerable error is introduced.
Apparently maintaining the dissimilar-metal junction at an elevated temperature for long periods varies the emf per .degree.C which is generated by the system, perhaps as a result of interdiffusion of the metals, diffusion of impurities from a furnace atmosphere into the junction or like changes in the sensor. The prolonged exposure to high temperatures may also effect the leads or conductors. These disadvantages are observed even when the system is enclosed in a ceramic sleeve.
As described in the above-identified applications, it has been proposed to avoid the disadvantages of conventional temperature-sensing systems by providing so-called noise-thermometers which utilize a different principle. A noise-thermometer system utilizes a metallic strand, wire, or film which generates an electrical output by thermal agitation of electrical charges within the conductor. The output is a noise voltage and is produced in the electrical conductor by such thermal agitation. Thermal noise, also known as JOHNSON noise, can be produced in a conductor even at temperatures approaching 0.degree.K at which thermocouples become noticeably less efficient, and may be particularly suitable for the measurement of temperatures in the range of several hundred .degree.K. The available thermal-noise power is proportional to the absolute temperature over the frequency band width over which the noise is measured, as described in the aforementioned applications. With a fixed band width, the available thermal-noise power can be measured in terms of the noise voltage and is proportional to absolute temperature. The theory of such systems and various circuits utilizing the principles of JOHNSON noise and temperature measurement are described in U.S. Pat. Nos. 2,710,899, 2,728,835, 2,768,266 and 2,884,786.
Frequently it is desirable to have available another temperature-measuring instrument with which a thermocouple can be calibrated with the aid of a noise thermometer or vice versa. For this reason two instruments are required and the introduction of both simultaneously to the measurment site may pose a problem. Furthermore, when reference to one and another indicator must be made repeatedly, the problem has been all but insurmountable with conventional systems.
In our earlier application Ser. No. 428,352 we disclosed a system having a single housing and insulating assembly, at least one thermocouple operating with a junction of dissimilar metals, and provided with the usual conductors or leads, a noise-temperature-measuring conductor connected to the leads of the thermocouple and preferably to the latter at its junction and returned by another conductor, and circuit means selectively connecting the thermocouple to a thermocouple temperature-measuring circuit and the noise-temperature sensor to a noise-thermometer circuit.
According to a preferred embodiment of this earlier invention, the noise-temperature sensor is connected at one end to a conductive shell, tube, or housing which encloses both the electrical resistor (noise-temperature sensor) and the thermocouple. Advantageously, two such thermocouples are provided with respective leads, and the noise-temperature-sensing resistor bridges the junctions of these two thermocouples.
Not only does the aforedescribed system provide the individual advantages of a noise thermometer and a thermocouple temperature sensor, but the overall system gives rise to new, useful and unexpected results which have not heretofore been obtainable. For temperature indication without concern for electrical disturbances in the system, the thermocouple is employed to provide an emf which may be rapidly and accurately converted into an indication of the temperature. To achieve this advantage the noise-temperature thermometer is used to calibrate the thermocouple and to check the determination of temperatures when using the thermocouple. The two sensors are so juxtaposed that differences in their operating temperature due to physical separation cannot occur, inasmuch as the noise-thermometer resistor is directly connected to the thermocouple junction.
Since a remote temperature measurement is possible, the parasitic effects resulting from long lengths of conductors can be canceled out or suppressed by utilizing the technique described in the aforementioned applications, especially since two conductors are provided for each thermocouple and may be used alternately to eliminate the variables resulting from such conductors.
The system according to this previous invention thus has the advantage that the two conductors may be used as described in the aforementioned applications to eliminate the resistance effects of the conductors extending to and from the measuring head. The protective tube also forms a shield for the system.
When a single thermocouple is employed and the conductive path to the noise-temperature thermometer is returned through the conductive shell, tube, or shield, the low resistance of the latter also renders the resistance of the return path negligible.